Organic electroluminescent (EL) devices (or organic light-emitting diodes (OLEDs)) are active emissive display devices based on the phenomenon that electrons combine with holes to emit light in a fluorescent or phosphorescent organic compound thin film when a current is applied to the organic film. Organic EL devices can be reduced in weight. Further, organic EL devices can be fabricated using a smaller number of components and parts in a simple manner. Further, organic EL devices have the advantages of high image quality and wide viewing angle. Other advantages of organic EL devices are fast response time that is suitable for dynamic images with high color purity, low power consumption, and low driving voltage. That is, organic EL devices have electrical properties suitable for use in portable electronic devices.
Novel phosphorescent materials and multilayer structures have been developed and introduced to increase the internal quantum efficiency of organic EL devices to 100%. Extensive research and development on new multilayer structures of organic EL devices has been focused on balancing holes and electrons in an emitting zone to improve the stability and characteristics of the organic EL devices.
Organic EL devices can be largely classified into organic EL devices (OLEDs) using low molecular weight materials and polymer EL devices (PLEDs) using high molecular weight materials according to the characteristics of the materials and the formation processes of organic films using the materials. For example, low-molecular-weight OLEDs are clearly distinguished from PLEDs in that organic films of low-molecular-weight OLEDs are mainly formed by vacuum evaporation whereas those of PLEDs are formed by spin-coating or printing.
Research has been conducted on new multilayer structures of PLEDs and proposals have been made that the structures are advantageous in terms of stability and efficiency of the devices. Unlike low-molecular-weight OLEDs, there are difficulties in fabricating PLEDs with a multilayer structure. The reason for the difficulties is that polymer films of PLEDs are formed by wet processing. For example, when a light-emitting layer as a primary thin film layer is formed on a substrate and a solution of a material for a secondary thin film layer (e.g., an electron injection layer) is applied thereto, the primary thin film layer is dissolved and slightly swollen by a solvent of the solution containing the secondary material. Therefore, it is important to choose appropriate solvents for the primary and the secondary materials for a multilayer structure in PLEDs. These problems arising from the use of solvents are substantially inevitable so long as a constituent material of a buffer layer between thin film layers is soluble in the solvents and has no cross-linkage, resulting in damage to the fundamental stability of PLEDs.
Therefore, the intermixing between primary and secondary thin film layers due to dissolution of the primary thin film layer by a solvent for the formation of the secondary thin film layer should be completely prevented with introduction of novel multilayer structures and this will be meaningful in improving the stability, luminescence efficiency and luminance of devices.